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DETECTORS Photo Ionization Detector - PID Overview The Photo Ionization Detector (PID) responds to all molecules whose ionization potential is below 10.6eV, including aromatics and molecules with carbon double bonds. The PID is nondestructive, so the sample can be routed through the PID and on to other detectors. It is often used in series with the FID and / or DELCD. Benzene PID detection limits for aromatics are in the ppb range; purge and trap concentration of the sample can lower detection limits to the ppt range. Because of its selective sensitivity, use of the PID is mandated in several EPA methods. The PID detector consists of a 10.6 electron volt (eV) UV lamp mounted on a thermostatted, low-volume (100µL), flow-through cell. The temperature is adjustable from ambient to 250oC. Three detector gain levels (LOW, MEDIUM and HIGH) are provided for a wide range of sample concentrations. The PID lamp is held in place by a spring-loaded plate, so that the lamp may be quickly removed for cleaning and replaced without any special tools. The PID can run on air carrier for gasless operation, or for stream monitoring applications where the entire stream of sample is directed through the detector (no column is used). 110 PID stand-alone detector PID in series with a DELCD and an FID on an SRI 8610C chassis Spring-loaded High voltage band in black plastic 100µL detector cell retaining plate hood over lamp anode ring (the area in the center of the TeflonTM seal ring) Heater block BNC connector to amplifier Collector electrode support union Amplifier gain switch Collector electrode DETECTORS Photo Ionization Detector - PID Theory of Operation 10.6eV PID Lamp Partial PID Assembly - The SRI PID design uses a 10.6eV lamp (SRI Part # 8670-1242) Exploded View with a high voltage power supply. Sample laden carrier gas flows from the analytical column into the PID sample inlet, where it is streamed through a 100µL flow-through cell. When sample molecules flow into the cell, they are PID lamp bombarded by the UV light beam. Molecules Anode ring with an ionization potential lower than 10.6eV release an ion when struck by the ultraviolet photons. These ions are attracted to a collector TeflonTM seal electrode, then sent to the amplifier to produce (SRI part # an analog signal, which is acquired by the 8670-1244) PeakSimple data system. Unlike other PID designs that heat the entire Cathode lamp, only the lamp window of the SRI PID is Collector base heated. This results in a longer lamp life for inlet SRI PID detectors. Simplified PID Operational Diagram NOTE: The end of the column must be visible in TeflonTM Spring-loaded the detector cell when the 100µL PID seal retaining plate PID lamp is removed from Ferrule detector cell the retaining plate. It should be approximately 1mm from Sample-laden Analytical the lamp window when the carrier gas column PID lamp is in place. inlet (from the column oven UV on 8610 & light 310 GC’s, or the heated transfer line on 110 models) High voltage band inside Ions are the black plastic hood attracted (must make contact with The PID cell effluent flows around the to the the lamp anode for PID column back to the PID sample gas collector operation; do not adjust outlet, which is connected to the next electrode unless the main GC power detector in series or vented to is turned OFF) atmosphere inside the column oven Collector electrode Collector electrode PID high support Heater block voltage union lead Column oven wall Collector electrode signal cable DETECTORS Photo Ionization Detector - PID Expected Performance PID Noise Run Column: 15m MXT-VOL Carrier: Helium @ 10mL/min PID gain: LOW PID temp: 150oC PID current: 70 PID noise averages less than 50µV from peak to peak PID BTEX Analysis (in series with FID and DELCD) Sample: 1µL 100ppm BTEX plus Column: 15m MXT-VOL Carrier: Helium @ 10 mLs/min PID gain: LOW PID temp: 150oC PID current: 70 Temperature program: Initial Hold Ramp Final 40oC 2.00 15.00 240oC 240oC 10.00 0.00 240oC Results: Component Retention Area Benzene 2.416 313.0540 TCE 3.066 231.5120 Toluene 4.600 309.2120 PCE 5.433 216.6230 Ethyl Benzene 6.700 286.0900 Ortho Xylene 7.383 298.9190 Bromoform 7.650 55.9460 Total 1711.3560 DETECTORS Photo Ionization Detector - PID General Operating Procedure The capillary column enters the PID cell from inside the column oven through the bulkhead fitting in the insulated oven wall. The column may be installed with the lamp in place. Insert the capillary column into the PID detector inlet until the column stops at the lamp window inside the PID cell, then pull it back about 1mm from the lamp window. Tighten the 1/8” nut with the graphite ferrule at the PID inlet to secure the column in place. The collector electrode is positioned at the factory and should not touch the column under normal circumstances. 1. Always ensure that the black plastic hood is in place on the lamp prior to operating the PID detector. The hood contains the high voltage band which is maintained at a high potential; never attempt to adjust the PID high voltage band unless the main GC power is turned off. The violet light is visible here when the lamp is on 2. Turn ON the GC. Turn ON the PID lamp current with the flip switch on the GC’s front control panel. 3. Set the PID current to 70 (= 0.70ma) with the trimpot setpoint on the top edge of the GC’s front control panel. Use the flat blade screwdriver provided with your GC to adjust the trimpot. The lamp should emit a violet-colored light visible down the center of the tube. PID - Close-up topview 4. Confirm that the lamp is operating at or near 0.70ma by pressing the PID detector ACTUAL display button on the front control panel. The sensitivity of the lamp increases proportionally to the current applied, but operation at higher currents reduces lamp life. The PID operating current range is 70-125. A setting of 70 should provide the user with sufficient sensitivity and lamp durability. Most PID applications can be performed using LOW gain. 5. Set the PID temperature to 150oC. 6. Once the detector has reached temperature and the signal appears stable, sample may be introduced. NOTE: Lamps are a consumable part of the PID detector. It is recommended to have a spare lamp available if critical analyses are being performed at remote field sites. Spare and replacement 10.6eV PID lamps are available under SRI part number 8670-1242. Teflon seals are available under SRI part number 8670-1244. DETECTORS Photo Ionization Detector - PID Troubleshooting and Maintenance Cleaning the PID Lamp The PID lamp window Over time, during normal operation, a film of contaminants will condense on the PID lamp window. Typically, this film is a result of stationary phase column bleed. To minimize contaminant condensation and thus lamp window cleaning, avoid heating the column any higher than absolutely necessary. Contaminant condensation can block the photons, reducing lamp emissions and sensitivity. Therefore, the PID lamp window must be cleaned when an appreciable change in sensitivity has been observed by the operator. Because the response change resulting from cleaning the lamp window usually requires detector recalibration, frequent cleaning is not recommended. 1. Turn the PID current OFF with the switch on the GC’s front control panel. Turn the GC OFF and let the PID detector assembly cool enough to touch it without getting burned. 2. Disconnect the high-voltage band from the lamp anode by removing the black plastic hood. 3. Grasp the spring-loaded retainer plate with the fingers of one hand and push or pull it toward the PID lamp; it doesn’t take much force to move the plate enough for lamp removal. Slide the PID lamp up and out of the PID detector assembly. 4. Clean the lamp window using a mild abrasive cleanser like Bon Ami or Comet. Wet your finger, and make a paste with a small amount of cleanser. Scrub the lamp window clean in a circular motion with your finger. 5. Rinse the lamp window clean with water. Dry the lamp with a paper towel. 6. Inspect the TeflonTM seal for cuts or nicks. A damaged seal will not affect the The collector electrode protrudes into the cell PID response, but it may provide a leak site that will reduce the amount of sample delivered to any subsequent detector. 7. With the lamp removed, the collector electrode is visible where it protrudes into the cell. Check the collector electrode for any visible residues, films, discolorations, etc. If present, they may impede the flow of ions from the sample molecules to the collector electrode. To clean the collector electrode, gently use TeflonTM seal a small file to remove any residues from its tip. Blow the residue off the collector electrode and surrounding areas. Make sure the lamp window is centered over the Teflon seal and snug against it 8. Open the spring-loaded retainer plate and replace the PID lamp snug against the seal. The lamp window has a slightly larger diameter than the seal; try to center it against the seal. Replace the high voltage band / black plastic lamp hood. 9. Recalibrate the PID detector before returning it to service.
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